Integrated circuits contain transistors that amplify signals with amplification factors that depend on the bias settings of the transistors. It is known to control the bias setting of transistors in an integrated circuit with a reference source. Typically, a reference source is a circuit designed to supply a voltage or current with a minimal dependence on uncontrollable factors such as circuit temperature.
US patent application No 2007/0075778 discloses an example of a reference source that uses two field effect reference transistors in a control loop that keeps the drain currents of the reference transistors equal. The sizes of the reference transistors differ by ratio of N:1 and the gate source voltages differ by an offset proportional to the drain current. As a result, the drain current is adjusted to a level where the effect of the transfer size ratio and the gate-source voltage offset on the drain currents are equal and opposite.
A resulting gate source voltage and/or the resulting drain current may be used as a reference to set the bias of amplifying transistors in the integrated circuit. In this way external influences on the reference are minimized. The reference depends on the characteristics of the reference transistors, the resistance used to derive the offset from the drain current and the ratio of the sizes of the reference transistors, which can be accurately controlled.
For amplifier circuits it is desirable that the small-signal gain of transistors is controlled. The reference source of US patent application No 2007/0075778 controls the gain to a certain extent, because it provides a controlled reference voltage to an amplifier. However, this does not prevent unpredictable variation of the small-signal gain due to factors that affect the current and the gain in different ways.